1. The Field of the Invention
The present invention generally relates to high speed data transmission systems. More particularly, the present invention relates to methods for identifying the frequency of light emitted by a laser and adjusting the laser operating conditions as necessary to maintain the laser emitting light at a desired frequency.
2. The Relevant Technology
The use of fiber optic technology is an increasingly important method of data transmission. Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities. Other advantages of using light signals for data transmission include their resistance to electro-magnetic radiation that interferes with electrical signals; fiber optic cables' ability to prevent light signals from escaping, as can occur with electrical signals in wire-based systems; and light signals' ability to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire. In practice, however, it is often necessary to convert an electrical signal to a light signal and vice versa.
Transceiver modules are widely used in the field of optoelectronics for this purpose. Typically, a transceiver module includes a transmitter optical subassembly (TOSA), a receiver optical subassembly (ROSA), and a printed circuit board (PCB) for coordinating the operation of each of the TOSA and the ROSA. Each of the TOSA and the ROSA may have an optical receptacle, for example an LC cable receptacle or an SC cable receptacle, at one end for attachment to an optical cable and some device at the other end to enable a conductive connection to a printed circuit board. The entire transceiver module, in turn, is connected to a computer system, such as a host system, for controlling the operation of the transceiver module. Thus, the computer system can direct the transceiver module to transmit an optical signal by directing an electronic signal through the printed circuit board and into the TOSA.
The TOSA in turn generates a modulated digital optical signal via an internal laser or light emitting diode (LED). The modulated digital optical signal typically includes pulsed light. The production of a pulse of light by the laser or LED can correspond to a digital “one” or “zero,” while no pulse corresponds to a “zero” or “one,” respectively, according to the configuration of the network. The modulated optical data signal produced by the laser can then be transmitted in a fiber optic cable via the optical network.
Similarly, the ROSA receives a modulated digital optical signal from the incoming optical cable at a photodiode, converts the optical signal into an electrical signal, and transmits the electrical signal to a PCB within the ROSA. From the PCB the electrical signal is relayed to a computer system.
The PCB includes, for example, a controller, which governs general operation of the transceiver, a laser driver for controlling operation of the laser in the transmitter portion of the transceiver, and a post-amplifier for controlling the conversion of incoming optical signals into electrical signals in the receiver portion. These components are typically configured as integrated circuits on the PCB.
One challenge in optimizing optical data transmission technology is the need to have precise control over the transmission or carrier wavelengths. Such control over the carrier wavelengths is necessary in order to provide stable communication. Problems in wavelength division multiplexing (WDM) systems, for example, occur when one or more of various multiplexed wavelength signals in an optical fiber begin to drift and thereby interfere with one or more multiplexed wavelength signals traveling on adjacent wavelength channels. These problems can be accentuated when the channel spacing becomes closer, for example with dense wavelength division multiplexing (DWDM) systems.
Wavelength drift can occur for a variety of different reasons. For example, wavelength drift can happen when optical elements, such as a laser, within a WDM system experience a temperature variation or a laser ages. Regardless of why the wavelength of a laser is prone to change, however, it is necessary to ensure that the laser emits at a relatively constant wavelength. Devices to adjust the lasing wavelength to a desired wavelength are known in the art, but to properly adjust the lasing wavelength it is nevertheless necessary to monitor emitted light and thereby know the actual wavelength of the emitted light. Devices external to a transceiver are often conventionally used to monitor the wavelength of a laser.
Certain devices and methods internal to a transceiver are also known for monitoring the wavelength of a laser. For example, U.S. Pat. No. 5,469,265 (“the '265 patent”) discloses various methods for determining wavelength. Among the disclosed methods is the determination of a ratio between unfiltered and filtered light, wherein the ratio can be used directly to ascertain the laser wavelength. Another disclosed method light uses a multiple quantum well electroabsorption device. In this embodiment, the multiple quantum well electroabsorption device is switched between reference and tracking modes to determine a ratio between the two. This has the disadvantage, however, of losing independent wavelength-dependent and wavelength-independent detector readings.
Another example of a device internal to a transceiver for monitoring wavelength is U.S. Pat. No. 6,243,401 (“the '401 patent”), which discloses the use of a semiconductor laser amplifier (SLA) as a wavelength discriminator. According to the '401 patent, the wavelength can be determined by measuring the transparent current of an SLA from the induced voltage across the diode junction when incident light is intensity-modulated. Based on the value of the transparent current, the wavelength is calculated by referencing a lookup table.
What is still needed, however, are improved and simpler methods for monitoring and controlling laser wavelengths.